In a clamping unit, and particularly in a chuck with exchangeable clamping jaws, one end or terminal position is reached during opening, when the clamping jaws approach the condition where stability is lost. The other end position is reached during closing, when the clamping jaws approach the point beyond which they cannot be moved further. Both end positions, however, should still be enough removed from a rest or stop position that a residual clamping stroke remains which insures safe clamping in or near the end positions. The final clamping positions differ only for chucks of different sizes, just as do the clamping strokes.
If a workpiece is to be clamped into a chuck, and the measurements of the workpiece exceed or fall short of the clamping range defined by the terminal clamping positions, proper clamping is no longer ensured, and there is the danger that when the machine starts up the workpiece or the chuck jaws will be flung away and cause damage.
For this reason, a device of the above-described type has already been developed (German published application 17 52 599) which determines whether the chuck jaws are still located within the range defined by the terminal clamping positions. If this is not the case, the machine cannot start.
In this known device, the sensors of the sensor device can be manually adjusted in relation to each other and in their spacing from a transmitter. The purpose of the adjustment is to enable adaptation to various chucks. However, it has been found in practice that faulty adjustment of the known device during a change of the chuck cannot be prevented by this device. The desired result is thereby placed in doubt.
The object of the present invention is to so improve the device of the above-described type that the safety risk is reduced and that an accidentally faulty adjustment is precluded.